Biggest tire under a stock suspension dart????

70 Dart, 8 3/4, Has 255 60 15s, with a 4.25 backspace. Could fit a 275 60 15 with fenders rolled but it would be tight (On my car atleast). So, just purchased a set of Mickey ET streets. 255 60 15s On a 8" rim with 4.5 backspace.